1. Field of the Invention
The present invention is related to a press working method and a press working apparatus in which a press-formed product is worked by a servo press.
2. Related Art
In a conventional mechanical press, press working is conducted when a slider, which is connected to a crank shaft, is driven upward and downward by a crank shaft rotating motor. In this type mechanical press, since a die is continuously moved upward and downward, it is difficult to change a speed of the die according to a working condition. Further, it is also difficult to temporarily stop the die.
Due to the above circumstances, a servo press has been recently used which can flexibly control a motion of a die with a servo motor. Concerning this matter, refer to JP-A-2001-150200 and JP-B2-3537287. In this type servo press, it is possible to stop the die at a predetermined position in the middle of one stroke in which the die is moved upward and downward.
In this connection, in the case of making a press-formed product, sometimes, a plurality of press forming steps are conducted according to a shape of the product. For example, the following are described in JP-B2-3537287. When a workpiece is worked with a press forming machine described in JP-B2-3537287, it is necessary to conduct multiple steps of press forming. Alternatively, in some cases, a compound working step, in which press working and another working are alternately, repeatedly conducted, is carried out.
However, according to JP-B2-3537287, multiple steps of the stopping positions of the upper die are set with respect to a workpiece which requires multiple steps (multiple motions). With respect to the other working which can not be executed only by the upper die, it is necessary to provide another independent working means, which can be a factor to increase the working time and working cost.
Especially, in the case of a plate for which a plurality of times of press forming are required, it is actually necessary to provide press forming apparatus, the number of which is the same as the number of press forming steps. Therefore, a space for installing the press forming apparatus is increased. Further, it takes labor to convey a plate between the apparatus.
Further, in press working, when the working is conducted while a periphery of a portion to be pressed is being pinched by a predetermined holder, it is possible to prevent the generation of wrinkles. In JP-A-2005-199318, a method is described in which working is conducted while a steel plate is being held by a holder.
In this connection, when the plate is held by the holder at the time of press working, depending upon a shape of a product to be formed, an excessively high tension is given to the plate concerned. Accordingly, more wrinkles are generated. Further, excessively small wall thickness portions may be generated on the product to be formed.
According to the method described in JP-A-2005-199318, the steel plate is lightly pushed by a blank corresponding to the holder. Therefore, at the time of working, the steel plate is drawn inside a die, that is, the steel plate is not continuously pushed by the blank at all times. Accordingly, it is possible to somewhat prevent wrinkles from being generated. It is also possible to somewhat prevent small wall thickness portions from being generated.
However, the method described in JP-A-2005-199318 is so-called deep drawing. Therefore, it is impossible to effectively apply the method described in Patent Document 3 to forming of mass-produced products.
The method described in JP-A-2005-199318 is applied on the assumption that almost all the steel plate is drawn inside the die being slid on the die at the time of working. Therefore, it is necessary that a pushing force given by the blank is finely adjusted according to the shape of the formed product and the material and the surface roughness of the steel plate. Further, an operator must be sufficiently experienced in selecting appropriate lubricant for press forming.